The present invention relates to a process for the production of silicone foams by means of ultrahigh-frequency waves.
Silicone foam materials have been known for some time for various areas of application via various production processes. The manifold areas of application include thermal insulation at elevated temperatures, the production of flexible seals, use as damping elements in the form of foams, inter alia. In these applications, use is made of the known properties of elastic silicone compositions: temperature stability, small change in mechanical properties at varying temperatures, good ageing stability, inter alia.
The various processes for the production of silicone foams are numerous and are used depending on the specific requirements.
Widespread use is made of silicone compositions containing silyl groups which simultaneously eliminate hydrogen during the crosslinking process. Suitable co-reactants are silanol groups, alcohols or even water. The hydrogen formed serves as blowing gas and generates the silicone foam having the desired pore structure, see, for example, EP-A 416 229. A further process is used in the production of silicone foam from heat-vulcanizing, peroxidically crosslinkable siloxane compositions. Here, substances which decompose at elevated temperatures are used as blowing agents which provide the expanded siloxane composition for the moment of vulcanization. This process has experienced widespread acceptance in industry and is described, for example, in U.S. Pat. No. 2,857,343.
Processes have also been described in which use is made of the gas solubility, for example in moisture-curing silicone compositions, in particular at elevated pressure. When the compositions are depressurized, the solubility drops suddenly, and the gas bubbles forming produce the desired silicone foam, which is then crosslinked by, for example, contact with water from atmospheric moisture, see, for example, U.S. Pat. No. 4,229,548.
Of these three systems, only the first allows the production of a solid, voluminous silicone foam; however, this satisfies only very limited demands regarding the mechanical properties, in particular with respect to strength and elasticity.
It has furthermore been attempted to use platinum-catalysed, addition-crosslinking silicone mixtures, in which, as is known, high elasticity and good rubber-mechanical properties can be achieved, to produce silicone foams and moulded silicone foam articles, see EP-A 751 173. The heat necessary to activate the blowing agent and for the vulcanization is supplied externally here, for example using a fan-assisted oven or, in the case of an injection moulding machine, using the heatable mould. A disadvantage here is that the foam volume which can be achieved is limited, since a heat-insulating effect sets in from the outside as the foam begins to form, hindering the passage of further energy inwards and thus excessively slowing or preventing further expansion and vulcanization in the core from a certain foam layer thickness, possibly after 2 to 3 cm. Thicker silicone foam boards or moulded shells, as could be used for thermal insulation at elevated temperatures, could therefore not be produced.
EP-A 497 565 has already described the production of silicone foams by means of ultrahigh-frequency (UHF) waves in the presence of azodicarboxamide as UHF-active substances. However, contamination of the crosslinking system through the use of azodicarboxamide and dinitrosopentamethylenetetramine has been observed (see U.S. Pat. No. 5,246,973).
There was therefore a need for a process for the production of voluminous silicone foams which does not have the disadvantages of the prior art.
Surprisingly, it has now been found that the above object can be achieved extremely well by means of addition- or condensation-crosslinking silicone compositions initiated by UHF waves if they contain certain carbonates and/or hydrogencarbonates and certain UHF-active substances. This combination allows the production, in a short time, of elastic, voluminous silicone foam elements.
The present invention relates to a process for the production of silicone foams by means of ultrahigh-frequency waves, characterized in that the addition- or condensation-crosslinking silicone compositions contain, as constituents, alkali metal and/or ammonium carbonates and/or alkali metal and/or ammonium hydrogencarbonates and UHF-active substances.
In a preferred embodiment of the invention, the addition-crosslinking silicone composition used is a mixture of
a1) 100 parts by weight of at least one vinyl group-containing linear or branched organopolysiloxane containing at least 2 vinyl groups and having a viscosity of from 0.1 to 1000 Pa.s,
b1) from 3 to 200 parts by weight, preferably from 5 to 50 parts by weight, of at least one, optionally surface-modified filler,
c1) from 0.5 to 10 parts by weight, preferably from 1 to 8 parts by weight, of hydrogensiloxane containing at least 3 SiH functions per molecule,
d1) from 0.01 to 100 ppm, preferably from 0.03 to 50 ppm, of platinum in the form of a platinum catalyst,
e1) from 0.01 to 5 parts by weight, preferably from 0.03 to 3 parts by weight, of an inhibitor.
The vinyl group-containing organopolysiloxanes (a1) are preferably linear or branched organopolysiloxanes containing at last 2 vinyl groups whose viscosity, measured at 20xc2x0 C. using a rotational viscometer, is in the range from 0.1 to 1000 Pa.s. Particular preference is given to vinyl-terminated polydimethylsiloxanes having a viscosity of from 0.2 to 150 Pa.s, optionally mixed with the dimethylsiloxanes containing pendant vinyl groups.
All viscosity data were measured at 20xc2x0 C. in accordance with DIN 53 019.
Examples of fillers (b1) are extender fillers, such as, for example, quartz sand or cristobalite flour and, precipitated or pyrogenic silicas, whose surface is preferably treated, before or during the mixing process, with substances known per se, such as, for example, silazanes, with or without addition of water.
Component b1) is preferably finely divided pyrogenic or precipitated silica, which has optionally been surface-modified with hexamethyldisilazane and/or tetramethyldivinyldisilazane.
For the purposes of the invention, component c1) comprises known polyorganosiloxanes carrying hydrogen atoms on at least three silicon atoms, such as, for example, compounds of the formula 
where R=C1-C8 alkyl or C6-C8 aryl,
m is=3, and
m+n=3xe2x88x921000, and where the xe2x80x94SiOR2 and xe2x80x94SiRHO units are randomly distributed in the molecule.
The Pt catalyst (d1) is preferably a Pt complex which catalyzes the adduction of Sixe2x80x94H groups onto vinylsiloxanes. Preference is therefore given to Pt(0) complexes containing vinylsiloxanes as ligands. However, it is also possible to use other metal complexes, such as, for example, Rh compounds. Preference is given to Pt(0) complexes containing vinylsiloxane ligands which are soluble in the siloxane polymers.
Suitable inhibitors (e1) are all compounds which permit a targeted reduction in the crosslinking rate, but do not cause irreversible damage to the catalyst. Particular preference is given to short-chain or cyclic polydimethylsiloxanes containing a plurality of adjacent vinyl groups on the silicon atoms, such as, for example, tetramethyldivinyldisiloxane, ethinylcyclohexanol and/or tetramethyltetravinylcyclo-tetrasiloxane as component c).
In a further embodiment of the invention, the condensation-crosslinking silicone composition used is a mixture of
a2) 100 parts of a linear or branched organopolysiloxane containing at least two silanol groups, where the viscosity is preferably in the range from 100 to 1,000,000 mPa.s,
b2) from 3 to 200 parts, preferably from 5 to 50 parts, of at least one filler which has optionally been surface-modified,
c2) from 1 to 15 parts by weight of a silane crosslinking agent selected from the series consisting of carboxamide-eliminating silanes of the formula
CH3Si(OC2H5)X2,
xe2x80x83where X is either C6H5CON(CH3)xe2x80x94 or 
xe2x80x83or
oxime-eliminating silanes of the formula 
where R3 is an alkyl or alkenyl radical having 1 to 4 carbon atoms, R1 and R2 can each be an alkyl radical having 1 to 4 carbon atoms or hydrogen, or R1 and R2 together are an alkylene radical having 4 or 5 carbon atoms, and where a can adopt the value 0 or 1, or
carboxylic acid-eliminating silanes of the formula
R4yxe2x80x94Sixe2x80x94(OCOR5)4xe2x88x92y
where y=0 or 1,
where R4 is a saturated or unsaturated hydrocarbon radical having 1 to 10 carbon atoms,
and R5 is a monovalent hydrocarbon radical having 1 to 10 carbon atoms.
The viscosity of the linear or branched organopolysiloxane a2) was measured at 20xc2x0 C. in accordance with DIN 53 019. The organopolysiloxane usually consists of dimethylsiloxane units, but other organic radicals may also be introduced. The organopolysiloxane is furthermore terminated by silanol groups, but other arrangements or more than two silanol groups per molecule can also be used.
The optionally surface-modified filler b2) is highly dispersed or precipitated silica and/or precipitated chalk and/or finely divided quartz sand. It is also possible for fibres, metal powder and/or plastic powder to be present. The optional surface treatment can be carried out using, for example, hexamethyldisilazane or dirmethyldichlorosilane. The BET surface area in m2/g has been taken from the manufacturer and is usually measured by N2 absorption.
The condensation-crosslinking mixture may also contain further additives. The most important of these are catalysts which regulate the crosslinking rate, examples which may be mentioned being organotin compounds, such as, for example, dibutyltin acetate, titanium compounds or zirconium compounds. Also customary, if desired, are adhesion promoters, such as, for example, aminoalkylalkoxysilanes or butoxyacetoxysilanes, plasticizing silicone oils or hydrocarbons, fungicides, flame inhibitors, stabilizers for oxidation and heat protection, such as, for example, iron oxides, and dyes.
Furthermore, the reinforcing fillers, such as silica or carbon black, and the extender fillers, such as silicates and quartz sand, can also be varied within broad limits.
The ammonium carbonate, ammonium hydrogencarbonate or alkali metal hydrogencarbonate used in accordance with the invention, where alkali metal is preferably Na or K, is added to both the condensation-crosslinking and the addition-crosslinking mixtures in finely divided form. The particle size is preferably=50 xcexcm, preferably=40 xcexcm, particularly preferably=20 xcexcm, where the mean particle size is determined by counting and averaging (number average) by means of a light microscope using a calibrated scale. The addition can either be in pure form as a powder with subsequent intensive mixing or by addition of a batch prepared by mixing the carbonate with, for example, a diorganopolysiloxane as per a).
The abovementioned carbonates and/or hydrogencarbonates in accordance with the invention are preferably used in an amount of from 0.2 to 10 parts by weight, preferably from 0.5 to 6 parts by weight, the amount being determined by the desired degree of foaming.
The UHF-active substances used are graphite, carbon blacks, preferably electroconductive blacks, iron oxide, magnesium oxide and/or aluminium hydroxide. In the case of the addition-crosslinking mixture, the UHF-active substances can also be used in the form of a water-containing filler and/or water-containing zeolite.
The UHF-active substances can also be further substances of sufficiently high dielectric constant and sufficiently large dissipation factor at the frequency used.
UHF wave (microwave) generators are widespread and customary in the rubber and plastics industry. It is also possible to use standard equipment produced for kitchen use. The microwave frequency is usually in the range from 1000xc2x7106 Hz to 5725xc2x7106 Hz, the frequency 2450xc2x7106 Hz being frequently used in industry. However, it is also possible to use high-frequency waves in the range from 13.56xc2x7106 Hz to 40.68xc2x7106 Hz.
The duration of the UHF wave treatment is highly dependent on the activity of the UHF-active substance used. It can be in the range from, for example, 10-20 sec up to 10-20 minutes or more, the input power per unit weight of product to be foamed being of crucial importance. It is also possible to use a power gradient and/or to combine microwaves with thermal energy.
Both the addition- and condensation-crosslinking compositions may also contain further additives, such as, for example, coloured pigments or additives for increasing the flame resistance, such as, for example, carbon black or TiO2.
In the case of addition-crosslinking silicone compositions, the mixture is usually prepared also [sic] dividing the premix into two components, one containing the platinum catalyst and the inhibitor and the other containing the hydrogensiloxane crosslinking agent. These two components can be stored separately. The admixture of the carbonate and the UHF-active substance can take place either after mixing of the two components or to one of the two components before mixing. The full mixture is then exposed to the UHF waves for foaming.
In the case of the condensation-crosslinking mixture, stirring-in of the carbonate and/or hydrogencarbonate is preferably delayed until just before the foaming. The UHF-active substance may already be present in the carbonate-free premix, but can also be incorporated together with the carbonate and/or hydrocarbonate together [sic] or just after the dienes.
The examples below, in which all parts are parts by weight, explain the invention without representing a limitation.